1. Field of the Invention
The present invention relates to a calibration device used for white calibration of an optical characteristic system measuring apparatus and an optical characteristic measuring system including this calibration device and the optical characteristic measuring apparatus and particularly to various spectrophotometers and color-difference meters of, e.g. so-called top port type or handy type suitably used as the optical characteristic measuring apparatus.
2. Description of the Background Art
A conventional optical characteristic measuring apparatus is described below, taking an apparatus of the top port type as an example. A typical prior art of the above top port type optical characteristic measuring apparatus was proposed in Japanese Unexamined Patent Publication 2002-243550 (document D1). A structure of a present spectrophotometer 100 of the top port type which was improved based on technology disclosed in document D1 is shown in FIGS. 15 and 16. FIG. 15 is a horizontal sectional view of the spectrophotometer 100 and FIG. 16 is a vertical sectional view thereof. This spectrophotometer 100 includes a measuring port (measuring opening) 103 which is so formed as to continuously extend through a ceiling plate 101a of a housing and a ceiling surface of an integrating sphere 102, and measures an optical characteristic such as color of a specimen 104 arranged to close the measuring port 103.
Thus, a xenon lamp 110 as a light source and an optical fiber 106 for measuring the light source are provided on one surface of the integrating sphere 102, a receiving optical system 108 is facing an opening 107 formed in one lateral surface, and a mirror 109 for forming an optical path between the measuring port 103 and the opening 107 is provided in a central part of the integrating sphere 102. Illumination light from the xenon lamp 110 is emitted into the integrating sphere 102 to be scattered in the integrating sphere 102 and illuminate the specimen 104 arranged on the measuring port 103. Reflected light from the illuminated specimen 104 is incident on the receiving optical system 108 through the opening 107 via the mirror 109, a reflected light intensity signal is obtained by a light receiver of the receiving optical system 108, and the color of the specimen 104 is measured by an arithmetic control unit based on the reflected light intensity signal. Further, the spectral intensity of the illumination light itself is obtained as a reference light intensity signal when the illumination light is incident on the receiving optical system 108 from the optical fiber 106 with the measuring port 103 closed.
Since being of the top port type, the spectrophotometer 100 constructed as described above can measure reflected light intensity by directly placing a specimen 104a such as a large fruit, which will substantially close the measuring port 103, on the measuring port 103 as shown in FIG. 17 and can measure reflected light intensities of specimens such as granules, powders and liquids using a transparent plate such as a dish 104c as shown in FIGS. 18 and 19A.
In the case of performing white calibration (calibration with an input of 100%) of such a spectrophotometer 100, a calibration device 111 shown in FIG. 20 has been conventionally used. As described above, since not only the specimen 104a is directly placed on the measuring port 103, but also the specimen 104b is indirectly placed thereon using the transparent plate such as the dish 104c, this calibration device 111 is composed of a white calibration plate 112 and a dummy transparent plate 113 formed to have the same material and thickness as the dish 104c. 
Calibrations include zero calibration (calibration with an input of 0%) performed with a dark-room environment set in the integrating sphere 102. In this case, a cylindrical zero calibration box 141 as shown in FIG. 21 is used. This zero calibration box 141 includes a cylindrical portion 142, which will surround the measuring port 103, and a conical member 143 provided to close the leading end of the cylindrical portion 142, and inside of the zero calibration box 141 is black which absorbs the light. This zero calibration box 141 scatters light incident through the measuring port 103 by the conical member 143 and absorbs the light by the inner circumferential surface of the cylindrical portion 142 having a predetermined length. In the case of this zero calibration, after the dummy transparent plate 113 of the calibration device 111 is firstly placed on the measuring port 103, the zero calibration box 141 is mounted thereon as shown in FIG. 19B and zero calibration is performed.
In the above background art, white calibration is performed with the white calibration plate 112 placed on the dummy transparent plate 113 as shown in FIG. 19C following zero calibration. Thus, in the above background art, an interference pattern 115 as shown in FIG. 22 may be formed due to a difference in flatness between the dummy transparent plate 113 and the white calibration plate 112. This interference pattern 115 differs depending on how the white calibration plate 112 is placed on the dummy transparent plate 113 and adversely affects white calibration accuracy.
Such an inconvenience could occur not only in apparatuses of the top port type, but also in optical characteristic measuring apparatuses of other types. For example, there are cases where a transparent member is mounted to cover a measuring opening to prevent entrance of powder or the like through the measuring opening, for example, in the case of measuring the powder or the like in a handy type apparatus. In white calibration performed prior to a measurement of the powder or the like using the handy type apparatus, a transparent member of the handy type apparatus is placed on a white calibration plate. Thus, an interference pattern may be similarly formed due to a difference in flatness between the transparent member and the white calibration plate, which adversely affects white calibration accuracy.